Borate application, redux

[chem geek]

In my post above "margin of error (MOE)" should be "margin of exposure (MOE)". It's too bad that I can't edit my posts after some time.

Yes, as shown in the Bioguard® Optimizer Plus® instructions, they recommend 50-80 ppm for biguanide pools. This is somewhat higher than the 50 ppm limit implied by Table 5 of the EPA report that has a 100 MOE for a roughly 350 mg/L sodium tetraborate pentahydrate product concentration which is equivalent to 350 * (4*10.8117/291.3) = 52 mg/L (ppm) Boron. I suspect that the biguanide dose rate is grandfathered.

As for the buffer explanation, I think it would be better to refer to the buffer as resisting moves in pH rather than moving the pH itself (at least for the buffer part of the explanation -- carbonates have a separate carbon dioxide outgassing effect that is not related to their buffering). The borates resist moves of pH in both directions, not just down, but since the resistance gets stronger as the pH rises it acts like a spring being compressed at higher pH just as the carbonate buffer acts as a spring being compressed at lower pH. So the carbonates resist the pH getting too low while the borates resist the pH getting too high even though technically they both resist pH changes in both directions at any given pH. A key point, however, is that the carbonates (represented by TA) are a SOURCE of rising pH in their own right, due to carbon dioxide outgassing, and that has nothing to do with their buffering capability -- it is an additional and unique effect that does not apply to borates. So substituting borates for carbonates as a buffer system makes sense to reduce a source of rising pH.